In recent years the need for global data networking capability has rapidly expanded. In order to meet this need, broadband satellite communication systems have been proposed as an alternative to land-based communication systems. One type of satellite data communication system is described in a number of U.S. patents assigned to the assignee of this patent application, including U.S. Pat. Nos. 5,386,953; 5,408,237; 5,527,001; 5,548,294; 5,641,135; 5,642,122, and 5,650,788. These patents and other pending applications assigned to the assignee of this patent application describe a satellite communication system that includes a constellation of low-Earth-orbit (LEO) satellites that implement an Earth-fixed cellular beam approach to transmitting data from one location on the Earth's surface to another location. In an Earth-fixed cellular beam system, the entire Earth is divided into a plurality of non-overlapping cells. Each of the cells are of similar predetermined size and are fixed with respect to the Earth.
More specifically, each LEO satellite has a communication footprint that covers a portion of the Earth's surface as a satellite passes over the Earth. The communication footprint defines the area of the Earth within which ground terminals can communicate with the satellite. Located within each footprint are a large number of the cells. During the period of time a cell remains within the borders of a satellite footprint, ground terminals located in the cell transmit data to and receive data from the "servicing" satellite.
As the servicing satellite moves through orbit, the footprint of the servicing satellite will move across the Earth. Thus, new cells at the leading edge of the footprint are added to the coverage of the servicing satellite. Similarly, cells at the trailing edge of the footprint are removed from the coverage of the servicing satellite. As the cells on the trailing edge of the footprint of the servicing satellite are dropped, another satellite in orbit is positioned to "service" each Earth-fixed cell previously covered by the satellite reaching the end of its servicing arc. During servicing, the antennas of ground terminals located in the cells continuously point toward the servicing satellite as it moves in orbit and antennas on the satellite point toward the cells. Other LEO satellite communication systems employ a satellite-fixed beam approach whereby the antenna beams from the servicing satellite remain fixed with respect to the satellite.
Regardless of whether an Earth-fixed cell system or a satellite-fixed beam system, data to be sent from one location on the Earth to another location is transmitted from a ground terminal located within a cell to the satellite serving the cell via an uplink data channel. The data is routed through the constellation of LEO satellites to the satellite serving the cell within which the ground terminal of the designated receiver is located. The latter satellite transmits the data to the receiver ground terminal via a downlink data channel. Thus, the constellation of LEO satellites and the ground terminals form a satellite data communication network wherein each ground terminal and satellite forms a node of the network.
In order to maximize the efficient use of the uplink and downlink bandwidth, various types of multiple access techniques may be used between the LEO satellites and the cells. One popular technique is frequency division multiple access (FDMA). In this well known technique, the available bandwidth is divided into a plurality of channels, each channel operating on a predetermined frequency range within the available bandwidth. See, e.g., J. D. Gibson, "The Mobile Communications Handbook", at 280-282, IEEE Press (1996); T. S. Rappaport, "Wireless Communications: Principles & Practice", at 397-400, Prentice-Hall (1996). Optimally, the assignment of frequency ranges to each channel is performed so as to minimize interference between adjacent channels (referred to as "cross-channel interference").
One shortcoming of a FDMA system is that FDMA is susceptible to co-channel or cross-polarization interference between cells that are using the same channel and that are communicating with the same satellite as the signal of interest. Indeed, co-channel interference will occur in any communications system that reuses frequency resources. To increase communication capacity per footprint, it is desirable to assign the channels in such a manner so as to minimize the spatial distance between cells using the same channel. Therefore, co-channel or cross-polarization interference will always appear.
One method of dealing with the co-channel interference is to design the satellite antenna to be extremely focused spatially, i.e., only have the antenna receive signals from the geographic area of the specified cell. The antenna gain pattern should have sidelobes that give minimal gain to signals outside of the main lobe. Thus, the antenna should have low sensitivity to signals that do not originate from a point of interest. While this can be done, it adds complexity and cost to the antenna.
Cross-polarization interference will occur in any communications system that uses bi-polarization schemes. Cross-polarization interference refers to the signal degradation caused by users within the same footprint that have been assigned the same channel as the signal of interest, but using an opposite polarization as the signal of interest. One method of dealing with cross-polarization interference is to design the satellite antenna to be polarization selective. In other words, the maximum axial ratios of the receive and transmit antennas must be tightly specified. However, the axial ratios of the antennas are not constant in all directions and generally increase with off-axis and scan angles. In addition, the received signals do not always retain their original polarization due to atmospheric phenomena. Additionally, manufacturing antennas that can maintain absolute orthogonality (i.e., infinitely low axial ratios) is difficult. Cross-polarization interference is particularly noticeable in the case of multiple users in the same cell as the signal of interest that use an orthogonal polarization scheme. This is because the only form of discrimination between such users is cross-polar isolation and there is no spatial discrimination available between these "cross-polar" users.
In order to minimize the complexity and cost of the satellite antenna, what is needed is a new method for efficiently reducing co-channel or cross-polarization interference in satellite communication systems, while maintaining a desired capacity density. Alternatively, the present invention may be used in conjunction with antenna design to further decrease co-channel or cross-polarization interference.